The purpose of this project is to study the cell surface membrane, how it may change in transformation to malignancy, either by spontaneous induction, or as induced by SV40 virus; to study the nature of biochemical and biologic changes associated with the cells being transformed by either SV40 or by spontaneous event(s). Families of clonal mouse embryo fibroblast cells were obtained in the past with very different tumorigenicity. These cells were either transformed by SV40 in culture, or by a spontaneous (unknown) event. A specific change in heparan sulfate biosynthesis on the cell surface was identified in both type of transformation; no change was detected in any other glycosoaminoglycan biosynthesis. In the SV40 transformed cells the SV40 T antigen is recognized as a virus (tumor) specific transplantation antigen (TSTA) in transplantation experiments in mice, and can lead to the rejection of all T antigen positive clonal cells. The tumor cells which grow then were all shown to be derived from a (mutant) cell which by a rare recombination event, lost a substantial portion of the early half of the SV40 DNA necessary for coding for functional T antigen and TSTA. The necessary and also the insufficient pieces of the SV40 DNA were identified by Southern blots, using appropriate DNA probes.